Pulse Density Modulation (PDM) is used to convert analog signals to binary data and vice versa, where bit rate or clock frequency are not limited. PDM is used in Sigma-Delta circuits to produce analog audio outputs (e.g. in a DAC embodiment) or to convert analog inputs to digital samples (e.g. in a ADC embodiment). Miniature digital MEMS microphones also use PDM as their digital output stage.
PDM uses a train of identical pulses of a single clock duration and constant amplitude, whose density describes the amplitude of a given analog signal. The pulse train can be regarded as a series of 1-bit samples at a sampling rate that is much higher than the Nyqvist sampling rate of the analog signal.
Most Digital Signal Processing (DSP) systems that use PDM input ports must convert the input data into multiple bit samples before making any processing. This conversion is usually done by dedicated hardware that includes complicated Sigma-Delta designs. For example, a beam-forming system that uses multiple digital PDM microphones would first convert all its PDM inputs into 16-bit samples, and then mix them together with individual delays and weighting factors. The weighting-factors will require a 16-bit multiply operation per every channel, and each delay may require a calculation of an interpolation filter (for sub-samples delays). This would consume a significant silicon area as well as significant amount of DSP MIPS.